TDD has the flexibility of adapting time resources between uplink and downlink transmissions. By dynamically changing the uplink/downlink (UL/DL) ratio to match the instantaneous traffic situation, the end-user performances can be improved. Another benefit of dynamic TDD is network energy saving, i.e. the improvement of downlink resource utilization allows an evolved NodeB (eNB) to configure downlink (DL) subframes more efficiently so that energy saving can be achieved.
In general, it is not preferable to change the UL/DL among macro sites at least not on a small time scale. However, for heterogeneous network, it is very likely that only a few user terminals are active simultaneously per local-area node, which implies a high possibility that many neighboring cells are momentarily empty. The traffic dynamics are expected to be large with relatively low average load but high instantaneous data rates. In this case the traffic asymmetry between uplink and downlink directions becomes more dominant which makes dynamic UL/DL configuration attractive.
In a dynamic TDD capable cell, both legacy UE, which does not support dynamic TDD, and advanced UE, which supports dynamic TDD can coexist. For a legacy UE, it is configured with just one TDD configuration which is in the (System Information Block) SIB message. SIB is a general term for this type of broadcast messages. There are a number of different SIB messages for different purposes and those may be numbered consecutively. For example, the TDD configuration may be transmitted in SIB1. In the following the terms SIB and SIB 1 is used interchangeably. For a dynamic TDD capable UE, the device is usually configured with one UL reference TDD configuration, and one DL reference TDD configuration. The actual TDD configuration used by an advanced UE varies within a TDD configuration group where the advanced UE's UL subframe is the subset of UL reference TDD configuration and DL subframe is the subset of DL reference TDD configuration. In order to obtain some benefit, in 3rd Generation Partnership Project (3GPP), it is agreed that the eNB notify dynamic TDD capable UE of the actual TDD configuration using explicit signaling. The explicit signaling could be in the form of a message signaled on the downlink physical control channel for example in a DCI on PDCCH.
The TDD configuration known by legacy UE can be different from dynamic TDD capable UE. Additionally, the TDD configuration known by dynamic TDD capable UEs can be different as well. Some dynamic TDD capable UE may know the up to date TDD configuration via explicit signaling. However, some dynamic TDD capable UE may instead know the old TDD configuration based on previous explicit signaling. This inconsistency of TDD configuration results in PUCCH HARQ resource conflict issues. For example, two UEs may transmit feedback on the same resource leading to problems with determining what was received by the base station. In this regard, when different UE have inconsistent TDD configurations, PUCCH HARQ resource conflicts result when different UE with different TDD configurations need feedback DL HARQ on the same UL subframe.
Furthermore, if a dynamic TDD capable UE did not detect the TDD configuration in the explicit signaling due to either bad radio channel quality or DRX, the eNB and UE will have different view on the TDD configuration to be used. For example, one issue, that arise when there is a different view on which TDD configuration to be used, relates to the PUCCH format 3 encoding/decoding as the PUCCH format 3 encoding/decoding is based on which TDD configuration is used. The PUCCH timing and resource mapping is also based on the used TDD configuration.